A roadside bomb in Syria has killed a local al Qaida leader and his wife, activists said, in an attack that could ignite new in-fighting between rebel groups in Syria.

The Britain-based Syrian Observatory for Human Rights said Ali al-Nuaimi of the Nusra Front, al Qaida's affiliate in Syria, and his wife were killed near the town of Busra al-Sham in southern Daraa province.

The Observatory said the killing was an "assassination", which came days after Nusra fighters seized a controversial Western-backed Syrian military commander, Ahmad al-Nuaimi. It is unclear if the two men are related.

It was not known if the two incidents are connected but the killing risks igniting rebel in-fighting in the south, between more moderate Syrian fighters and the hard-line Nusra Front.